Various types of sheet conveying rollers are incorporated in a sheet conveying mechanism provided, for example, in an image forming apparatus such as an electrostatic copying machine, a laser printer, a plain paper facsimile machine, a copier-printer-facsimile multifunction machine or an inkjet printer, or machinery such as an automatic teller machine (ATM).
Examples of the sheet conveying rollers include a sheet feed roller, a transport roller, a platen roller and a sheet output roller, which are each adapted to be rotated in frictional contact with a sheet (the term “sheet” is herein defined to include a paper sheet, a plastic film and the like, and this definition is effective in the following description) to convey the sheet.
Such a conventional sheet conveying roller typically includes a tubular roller body made of an elastic material such as a crosslinking product of a rubber and having an outer peripheral surface serving as a contact surface to be brought into contact with a sheet.
However, paper dust and dirt generated from paper sheets are liable to adhere to the outer peripheral surface of the roller body. As the roller body is repeatedly brought into contact with paper sheets, the paper dust and the dirt are accumulated on the outer peripheral surface. This reduces the contact area and the friction coefficient of the outer peripheral surface with respect to a paper sheet, thereby causing a sheet transportation failure at a relatively early stage.
A known effective method for maintaining the friction coefficient at a proper level for a longer period of time from the initial stage of use to prevent the sheet transportation failure is to form a plurality of linear projections or grooves parallel to each other axially of the roller body on or in the outer peripheral surface of the roller body. For optimization of such an effect, various configurations for the linear projections and the grooves are contemplated, and used in practical applications.
Particularly, a sheet conveying roller (generally referred to as “K-type knurled roller”) is generally widely used, which includes a plurality of linear projections arranged circumferentially equidistantly on an outer peripheral surface of a roller body thereof and each having a cross section such as defined by a normal distribution curve as taken perpendicularly to the axis of the roller body.
Further, a sheet conveying roller (generally referred to as “angularly knurled roller”) is also generally widely used, which includes a plurality of grooves formed circumferentially equidistantly in an outer peripheral surface of a roller body thereof as each having a generally V-shaped or U-shaped cross section taken perpendicularly to the axis of the roller body as shown in FIGS. 2 to 4 in Patent Literature 1.
The former K-type knurled roller effectively suppresses the reduction in friction coefficient which may otherwise occur due to adhesion of paper dust and dirt particularly when the roller is repeatedly used. Further, the latter angularly knurled roller effectively improves the friction coefficient at the initial stage of use.
Further, Patent Literature 2 discloses a serrated sheet conveying roller including a roller body having a plurality of linear projections which each have a generally triangular cross section taken perpendicularly to the axis of the roller body and having first and second edges circumferentially inclined at different inclination angles and are circumferentially equidistantly arranged in a state such that a first inclined edge of one of each two adjacent linear projections is directly connected to a second inclined edge of the other linear projection as seen in section.